The ability to fabricate semiconductor devices with sub-micron features has led to the reduction of the size of a die (a discrete unit that constitutes a semiconductor device) that can be manufactured. As a result, a greater number of die can be obtained from a single semiconductor wafer, leading to significant cost reduction.
According to the prior art, identical power semiconductor devices are formed in a semiconductor wafer, and then singulated into individual, discrete die. A conventional wafer for the manufacture of a power semiconductor device includes a silicon substrate of one conductivity, which is cut out of a larger single crystal silicon ingot, and an epitaxially grown silicon layer formed over one surface of the silicon substrate. According to conventional technology, the epitaxial silicon layer is doped with dopants of a selected conductivity (typically the same as the conductivity of the substrate) while the epitaxial silicon layer is being grown. The latter step will be referred to hereafter as in situ doping.
Many types of devices can be formed in a conventional wafer. A power semiconductor switching device which can be formed in a conventional wafer is a trench type power MOSFET. Trench type power MOSFETs are particularly suitable for high voltage and high current applications. A trench type power MOSFET is characterized by trenches that extend through a channel region in the semiconductor die, and support gate structures adjacent to the channel region.
According to a conventional design, the trenches of a trench type semiconductor power device are formed in the epitaxial silicon layer of a die. In a typical device, the epitaxial layer also includes source regions of a first conductivity type, a channel region of a second conductivity type, and an underlying drift region of the first conductivity.
In manufacturing semiconductor devices, it is generally desirable to use larger diameter wafers so that more discrete devices can be simultaneously formed. The use of large diameter wafers can, however, result in difficulties in obtaining critical device parameters. Some of these difficulties are caused by slight deviations in the dopant concentration across the body of the wafer. For example, a slight deviation in substrate temperature during in situ epitaxial growth, or during drive-in or activation of the dopants in the epitaxial layer, can result in deleterious non-uniformity in epitaxial thickness or doping levels. As a result, critical device parameters may become difficult to obtain when a conventional large diameter wafer is used, thereby reducing yield and increasing cost.